My PowerMac G4 plays the startup sound through its output speakers. It's a 2.1 setup with a 10lb (not too much of an exaggeration) woofer that isn't that great, but ridiculously loud and good for music and games (voices in movies always take a backseat to explosions and what not, really annoying, but I got them for free). Any-hoo, if I forget to mute the sound or accidentally leave the speaker's independent volume too high, the startup sound will shake the walls in my room. Scares the **** out of my every time.
With my 2008 MacBook, however, I usually have headphones plugged into it and it really bothered be at first that the chime always came through the internal speakers.